<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }
    .wrapper {
      width: 50vw;
      height: 100vh;
      background-color: grey;

    }
    .left {
      width: 200px;
      background-color: red;
      float: left;
    }
    .right {
      display: flow-root;
      height: 100px;
      background-color: green;
    }

  </style>
</head>
<body>
  <div class="wrapper">
    <div class="left">left</div>
    <div class="right">right</div>  
  </div>
</body>
</html>